In recent years, sealed secondary batteries each in the form of a flat rectangular box have become widely used in portable electronic apparatuses and the like. In one such secondary battery, a battery container normally comprises an open-topped metallic container body and metallic sealing plate. The container body is in the form of a flat rectangular box. The sealing plate is welded to the container body so that it closes an opening of the container body. Battery elements are arranged in the container body before the sealing plate is welded to the container body. Thereafter, the sealing plate is welded to the container body to form the sealed battery container. Then, an electrolyte is injected into the container body through an injection hole formed in the sealing plate. When the injection of the electrolyte is finished, the injection hole is sealed with a sealer in a reduced-pressure ambience.